With fuel cells being desired for automotive purposes it has become important for fuel cells to achieve full power quickly. Subfreezing fuel cell startups have difficulty quickly reaching an appropriate power level.
Electrochemical fuel cells convert fuel and oxidant into electricity, a reaction product (such as water in the case of a hydrogen fueled and oxygen oxidizing fuel cell) and heat. The fuel cell typically has a membrane electrode assembly (“MEA”) separating the fuel from the oxidant and the MEA is where the reactions take place. This membrane typically contains a catalyst and needs to be hydrated in order to function. The water to hydrate the membrane is formed on the oxidant side and can accumulate if the stack is operated at over 100% relative humidity. For subfreezing conditions this accumulated water can hinder fuel cell starts and can cause damage to the fuel cell. For this reason, an automotive fuel cell is sometimes dried using a gas purge when the system is shutdown. In order to be effective in each of the multiple cells which comprise the fuel cell stack, the gas purge must remove sufficient amount water while leaving the membrane enough hydration to allow a fuel cell to start. If the purge is not effective it leaves cells either too dry or too wet.
One of the problems if a fuel cell is left too wet is either side of an MEA can be covered with ice, preventing fuel and oxidant from reaching and reacting at the MEA. This would prevent the fuel cell from starting. Even if the MEA is only partially covered the water produced can freeze and cover the rest of the MEA if insufficient heat is produced to thaw out the cell. In any case ice covering part of the MEA will slow a fuel cell startup.
Another problem with a subfreezing start is that water can freeze and block the fuel or oxidant supply. If the water freezes and blocks the channels that are supposed to remove the water the fuel cell will flood stopping either the fuel or oxidant from reaching the membrane and stopping the fuel cell operation.
One way of making sure a fuel cell can start in subfreezing temperatures is to have it purge its channels at shutdown so that ice cannot clog the channels or cover the membrane and prevent the fuel cell from starting.
If the fuel cell is too dry the membrane conductivity will be insufficient to allow the fuel cell to operate at large loads. In this case a rapid start cannot be achieved and operation will be limited until the stack temperature rises and or the membrane becomes sufficiently hydrated during operation.
During a fuel cell start power is requested from the stack based on the demands of the system. This power is achieved by controlling the current density during the start based on cell voltage. The cell voltage is a function of current density and the membrane conductivity which is controlled by temperature and hydration. To meet the power request a current density will vary over time as temperature and cell hydration increase. However, due to system constraints, the maximum current density will be limited based on the minimum cell voltage at which the system can operate. The objective of a start is to reach the requested power request as quickly as possible while satisfying such system constraints. Based on the starting conditions, the current density during the start can be optimized to achieve this. A current density time profile is how the current density is going to be varied over time during startup. The ramp rate is how quickly the current density time profile is going to increase.